Not applicable.
1. Field of Invention
This invention relates to a process for production of aliphatic esters by catalytic conversion of the corresponding alcohols having carbon numbers between 1 and 10. In particular, the process can be used advantageously for production of ethyl acetate by conversion of ethyl alcohol.
2. Description of Prior Art
Ethyl acetate is a commercially important chemical. It is especially suitable as a solvent for extraction processes in the food industry. It finds applications as a high-grade defatting detergent. It is used for preparation of cosmetics, glues, lacquers, paint as well as polymer solution in paper industry. High purity ethyl acetate is used as an anhydrous medium and also as an intermediate in chemical syntheses.
Commercially, ethyl acetate is recovered as a by-product or produced through chemical synthesis. In 1988, 65 and 35% of ethyl acetate produced in the US were from by-product recovery and chemical synthesis, respectively.
Ethyl acetate is recovered as a by-product from n-butane liquid phase oxidation and as a co-product in polyvinyl butyral production process.
As the demand for ethyl acetate increases due to environmental concerns, more ethyl acetate has to be produced through chemical syntheses. Currently, there are two commercial processes for synthesizing ethyl acetate, namely, the Hoechst process based on the Tischenko reaction and the esterification process based on direct reaction of acetic acid with ethanol. In the Tischenko reaction, acetaldehyde is dimerized to ethyl acetate in the presence of aluminum ethoxide. In the direct esterification process, ethanol is reacted with acetic acid in the presence of acidic catalyst.
In the Hoechst process, a catalyst solution of aluminum ethoxide is first prepared by dissolving granular aluminum in an ethanol-ethyl acetate mixture in the presence of aluminum chloride and a small amount of zinc chloride. The reaction evolves hydrogen and is exothermic. Intensive cooling is required to prevent the loss of organic matter. The final solution contains about 2% aluminum. The next step in the process is to introduce the catalyst solution along with acetaldehyde simultaneously into a reactor. The reaction varies according to the temperature and the catalyst quantity. These parameters are adjusted to accomplish about 98% conversion in one pass through the reactor. A further 1.5% transformation is obtained in the stirring vessels where a residue is separated from the product. The reactor is kept cooled to about 0xc2x0 C. by the use of a chilled brine. The residence time in the reactor is about one hour. The distillable products are removed in the residue separation vessel by evaporation. The residue is treated with water to convert as much as possible to ethanol. The remainder can either be treated in a biological degradation plant or incinerated. The combined distillable products are then separated in a series of distillation steps to give ethyl acetate, the product; unconverted acetaldehyde for recycle: light ends which can be used for fuel; a mixture of ethyl acetate and ethanol, which can be used in the catalyst preparation step; and a by-product, acetaldehyde diethyl acetal, which can be recovered for sale or hydrolyzed for recovery of acetaldehyde and ethanol. This process is complicated in operation procedures and equipment in the processing steps. It employs expensive and dangerous catalyst systems.
In the esterification process, ethanol and acetic acid are combined with a recycle of crude ethyl acetate in a reactor, which is also an azeotropic distillation column. The reaction produces water as a waste product. The water impedes the reaction, and the reaction column removes the water as an azeotrope as it is generated. The overhead condensate is collected in a decanter where the product separates into two phases. The organic phase is partially recycled to the reaction column and the balance is fed to a second distillation column, which produces a bottom product of ethyl acetate and an overhead product of an azeotrope of ethyl acetate, water and ethanol. The overhead condensate is collected in a second decanter, where it separates into two phases as before. The organic phase is recycled to the column while the aqueous phase is combined with the aqueous phase from the first column and fed to a third column to produce a waste stream from the bottom and the azeotrope from the top. The azeotrope is recycled to the reaction column. Since esterification is a reversible reaction, the conversion per pass is theoretically limited by the equilibrium constant K which is defined as follows: K=[ETOAC]xc3x97[H2O]/[ETOH]xc3x97[HOAC], where [ETOAC], [H2O], [ETOH] and [HOAC] are mole % of ethyl acetate, water, ethanol and acetic acid in the reactor, respectively. In order to overcome this problem, several azeotropic distillation columns with recycle as well as catalytic distillation can be used, leading to complicated and expensive operation. Furthermore, water in the ethanol feed impedes the reaction and limits conversion level of the esterification so that an expensive ethanol of low water content has to be used.
Ethyl acetate is synthesized from ethylene and acetic acid. U.S. Pat. No. 4,275,228 disclosed that ethyl acetate is prepared by vapor phase reaction of ethylene and acetic acid in the presence of a catalytic amount of a solid, ion-exchange fluoropolymer comprising sulfonic acid moieties. Conversions of acetic acid vary from 30% at 126xc2x0 C. with a residence time of 55 hours to 60% at 150xc2x0 C. with a residence time of 30 hours. Obviously, the reaction rate is too low to be commercially viable. In addition, the ion-exchange resin catalyst in the high temperature, oxidative reaction condition would itself be oxidized leading to rapid aging and crumbling. For the similar process, U.S. Pat. No. 5,241,106 reveals a variation whereby the catalyst comprises tungstophosphoric acid of which 10-90% of the total amount of proton is replaced with a member selected from the group consisting of (a) cesium metal ion, (b) a combination of cesium metal cation and at least one cation selected from alkali metal cations other than cesium cation, and (c) a combination of cesium metal cation and at least one cation of iron group metal cations. The process is flexible and can be carried out in either vapor or liquid phase. In addition, the reaction rate can be improved by adding control amount of water in the feed. However, the reaction rate and the yield of ethyl acetate remain too low for commercial application.
Ethyl acetate is produced by hydrogenation of acetic anhydride. U.S. Pat. No. 4,886,905 disclosed a process for preparation of ethylene diacetate and/or ethyl acetate by hydrogenating acetic anhydride in the presence of a homogeneous ruthenium catalyst, methyl iodide and, optionally, lithium iodide. The process can also be utilized to hydrogenate mixtures of acetic anhydride and ethylene diacetate to produce ethyl acetate alone. This process is complex to operate. It requires the use of homogeneous complex catalyst. The co-product, acetic acid, must be separated and converted back to acetic anhydride for reuse. In addition, the catalyst and iodides must be separated from the reaction products and recycled.
Another approach is to produce ethyl acetate from methyl acetate. In U.S. Pat. No. 4,780,566, a process is described for producing ethyl acetate either alone or in mixture with acetic acid, by homologation of methyl acetate with CO and H2 in the presence of a catalyst in the form of a Ru compound and catalysis promoter of hard acid type in an atmosphere of hydrogen and carbon dioxide. This process is not only complex but also poor in selectivity for ethyl acetate. It produces significant amount of by-product, acetic acid, alcohols, ethers, propionates, methane and ethane.
Ethyl acetate is produced by catalytic oxidation of ethanol. U.S. Pat. No. 5,334,751 disclosed a process for making ethyl acetate which comprises reacting in a reaction zone ethanol and molecular oxygen in the presence a solid catalyst containing the elements and protons indicated by the empirical formula, Pd(a)M(b)TiP(c)O(x) where M is selected from Cd, Au, Zn, Tl, alkaline earth metals. In addition, the catalyst contains crystalline TiP(2)O(7). The process is simple but might be difficult to avoid temperature run-away. In addition, the reaction temperature is so high that significant by-product is produced and purification of the product becomes difficult. The selectivity for ethyl acetate is poor and co-produces up to 75.5% of acetic acid, which has to be separated and sold. Further more, the presence of high level of acetic acid and water in the system causes corrosion of equipment and, particularly destruction of the catalyst.
In U.S. Pat. No. 5,770,761, a process is disclosed for oxidation of liquid ethanol in the presence of excess liquid ethanol and supported oxidation catalyst to produce ethyl acetate in one step. In the process, two chemical reactions are involved and two catalysts are used. For oxidation portion of the process, metallic oxidation catalyst on a hydrophobic support is preferred. For the esterification portion of the process, an acidic solid ion exchange resin is preferred. In the process, the selectivity for ethyl acetate is low so that the yields of ethyl acetate was between 10 and 40% and mostly around 20%, while the by product, acetic acid is around 10%. The low selectivity and yield of ethyl acetate makes the process inefficient and costly. As in a typical oxidation process, a significant amount of unknown by-product is co-produced in the process to complicate the product purification operation. The types and the amount of by-product increase greatly as the temperature is increased to increase the conversion levels of ethanol. It makes production of valuable, high purity ethyl acetate difficult and costly. The by-products also destroy the catalyst. As Lin et al. showed in the article published in IandEC Research Vol.38, No.4, page 1271-1276, 1999 that significant amount of by-products in the system, particularly, acetic acid causes loss of the Pd metal through a leaching process. They also cause migration of Pd leading to loss in catalyst activity and selectivity for ethyl acetate. The resins used as the oxidation catalyst support and acidic ion exchange catalyst disintegrate in the oxidative environment of the process. In the reaction system, said resins are oxidized itself leading to aging and crumbling. Thus further improvement is required to make the process technically and economically viable.
The prior art for production of aliphatic esters, particularly, ethyl acetate suffers from a number of disadvantages including:
a. Ethanol conversion levels are low leading to low yield of ethyl acetate. When severity of the process is increased by, for example, increasing temperature to increase the conversion of ethanol, copious amount of CO2 will be produced due to over oxidation.
b. Its selectivity for ethyl acetate is low and it co-produces significant amount of acetic acid. In a liquid phase operation, co-production of acetic acid in a significant amount is inevitable because the esterification step is limited by thermodynamic equilibrium. The presence of high level of acetic acid lead to corrosion of the equipment and damage to the catalyst.
c. The resins used as the oxidation catalyst support and acidic ion-exchange resin catalysts are destroyed in the oxidative reaction environment. The resins are oxidized themselves leading to aging and crumbling. The oxidation and destruction of the resin is further accelerated when it is loaded with metal component for use as an oxidation catalyst. Thus, the resin-based catalyst cannot be used in oxidation process for extended period of operation time,
d. In the liquid phase operation, the by-product, acetic acid damages the catalyst. It leaches out the metal such as Pd leading to loss of catalyst activity. The presence of acid also causes agglomeration of metals such as Pd on the support to deactivate the catalyst.
e. The by-product, acetic acid not only lowers conversion of ethanol per pass but also requires complicated system for separation and extensive recycling.
f. The non-selective oxidation involved in the process produces excess amount of CO2 causing loss of raw material, and significant amount of impurities causing difficulties in purification of ethyl acetate product.
g. The reaction conditions of the process are severe in order to increase the conversion level of ethanol. Thus, high temperature and pressure conditions are employed.
h. The significant amount of by-products from the oxidation process such as acetaldehyde and acetic acid have to be disposed or recovered for sale. This need causes environmental concerns and complicates the production of ethyl acetate.
There is a need in the industry for a more efficient and earth friendly process for the production of ethyl acetate.
The disclosures of the above patents and literature are incorporated herein by reference.
The present invention provides a process for production of aliphatic esters, R""COOR by reacting the corresponding alcohol, ROH with molecular oxygen in the presence of a dual functional catalyst comprising metal on acidic support. The carbon numbers of the alkyl chains, Rxe2x80x2 and R are 0 to 9 and 1 to 10, respectively. In particular, the process is employed advantageously to produce ethyl acetate by conversion of ethanol. The reaction mixture from the reactor is separated through azeotropic distillations in three columns to recover ethyl acetate product and by-product, acetaldehyde and acetic acid for recycle. The preferred catalyst is Pd on acidic Zeolite Y. It is active for ethanol conversion, selective for ethyl acetate and can regenerated by air oxidation. The reaction can be operated in either vapor or liquid phase but vapor phase operation is preferred for production of ethyl acetate. The process is characterized by high conversion of ethanol, high selectivity for ethyl acetate, low waste generation and stable operation.
Accordingly, several objects and advantages of the present invention are:
a. To provide a process for conversion of an alcohol, ROH to an aliphatic ester, Rxe2x80x2COOR, particularly, ethanol to ethyl acetate at high conversion levels and high yield of the ester, particularly, ethyl acetate.
b. To provide a process for production of ethyl acetate from ethanol with high selectivity for ethyl acetate and low selectivity for acetic acid.
c. To provide a dual functional catalyst which is stable in the oxidative reaction environment.
d. To provide a dual functional catalyst which is stable in activity and integrity at the reaction environment of present invention for extended period of operation time.
e. To provide a process which breaks the thermodynamic equilibrium limitation to minimize production of acetic acid.
f. To provide a process for converting an alcohol to ester which minimizes production of CO2 and impurities.
g. To provide a mild process for ester production at low pressure and low temperature.
h. To provide an earth friendly process for ester production which minimizes waste generation.
Further objectives and advantages will become apparent from a consideration of the ensuing description and drawing.